Physical Separation Processes

Physical separation techniques separate a mixture such as a crude oil without changing the chemical characteristics of the components. The 

The important physical separation processes, discussed here, are distillation, absorption, adsorption, and solvent extraction. 

Atmospheric distillation separates the crude oil complex mixture into different fractions with relatively narrow boiling ranges. In general, separation of a mixture into fractions is based primarily on the difference in the boiling points of the components. In atmospheric distillation units, one or more fractionating columns are used. 

Distilling a crude oil starts by preheating the feed by exchange with the hot product streams. The feed is further heated to about 320°C as it passes through the heater pipe (pipe still heater). 

The hot feed enters the fractionator, which normally contains 30-50 fractionation trays. Steam is introduced at the bottom of the fractionator to strip off light components. The efficiency of separation is a function of the number of theoretical plates of the fractionating tower and the reflux ratio. Reflux is provided by condensing part of the tower overhead vapors. Reflux ratio is the ratio of vapors condensing back to the still to vapors condensing out of the still (distillate). The higher the reflux ratio, the better the separation of the mixture. 

1 INTRODUCTION AND CLASSIFICATION OF MECHANICAL-PHYSICAL SEPARATION PROCESSES 

In Chapters 10 and 11 gas-liquid and vapor-liquid separation processes were considered. The separation processes depended on molecules diffusing or vaporizing from one distinct phase to another phase. In Chapter 12 liquid-liquid separation processes were discussed. The two liquid phases are quite different chemically, which leads to a separation on a molecular scale according to physical-chemical properties. Also, in Chapter 12 we considered liquid-solid leaching and adsorption separation processes. Again differences in the physical-chemical properties of the molecules lead to separation on a molecular scale. In Chapter 13 we discussed membrane separation processes where the separation also depends on physical-chemical properties. 

All the separation processes considered so far have been based upon physical-chemical differences in the molecules themselves and on mass transfer of the molecules. In this way individual molecules were separated into two phases because of these molecular differences. In the present chapter a group of separation processes will be considered where the separation is not accomplished on a molecular scale nor is it due to the differences among the various molecules. The separation will be accomplished using mechanicaLphysical forces and not molecular or chemical forces and diffusion. These mechanical-physical forces will be acting on particles, liquids, or mixtures of particles and liquids themselves and not necessarily on the individual molecules. 

The mechanical-physical forces include gravitational and centrifugal, actual mechanical, and kinetic forces arising from flow. Particles and/or fluid streams are separated because of the different effects produced on them by these forces. 

These mechanical-physical separation processes are considered under the following classifications. 

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In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic 2eohte, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes ahquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of Hquid to various sections of the adsorber (93,94). The cis- and trans-isomers are alkylated and used in the gasoline blending pool. 

Solvent separation, using the propane deasphalting process, is another procedure by which asphalts of the straight reduced type may be manufactured. This is a physical separation process used to recover high viscosity lube fractions from a given vacuum residuum. When mixed with the residuum, the solvent preferentially dissolves the oil and precipitates the asphalt. 

That benzene hexachloride isomer mixture is then the raw material for lindane production. The production of lindane per se is not a chemical synthesis operation but a physical separation process. It is possible to influence the gamma isomer content of benzene hexachloride to an extent during the synthesis process. Basically, however, one is faced with the problem of separating a 99%-plus purity gamma isomer from a crude product containing perhaps 12 to 15% of the gamma isomer. The separation and concentration process is done by a carefully controlled solvent extraction and crystallization process. One such process is described by R.D. Donaldson et al. Another description of hexachlorocyclohexane isomer separation is given by R.H. Kimball. 

The area of interest covered by this paper is limited to processes in which chemical conversion occurs, as in the processes noted above. Gas-liquid-particle processes in which a gaseous phase is created by the chemical reaction between a liquid and a solid (for example, the production of acetylene by the reaction between water and carbide) are excluded from the review. Also excluded are physical separation processes, such as flotation by gas-liquid-particle operation. Gas absorption in packed beds, another gas-liquid-particle operation, is not treated explicitly, although certain results for this operation must necessarily be referred to. 

In the thermal desorption technique excavated soil is heated to around 200 to 1000°F (93 to 538°C). Volatile and some semivolatile contaminants are vaporized and carried off by air, combustion gas, or inert gas. Off-gas is typically processed to remove particulates. Volatiles in the off-gas may be burned in an afterburner, collected on activated carbon, or recovered in condensation equipment. Thermal desorption systems are physical separation processes that are not designed to provide high levels of organic destruction, although some systems will result in localized oxidation or pyrolysis. 

The Maxymillian Technologies, Inc. (formerly Clean Berkshires, Inc.), mobile thermal desorption system (TDS) uses rotary kiln technology to remove contaminants from excavated soils and sediments. Thermal desorption is a physical separation process designed to volatilize water... 

The SoilTech anaerobic thermal processor (ATP) technology is a physical separation process that thermally desorbs organics such as polychlorinated biphenyls (PCBs) from soil and sludge. The SoilTech system distills organic contaminants from a solid matrix in an anaerobic environment, thus preventing oxidative degradation of contaminants such as PCBs into more harmful reaction products. Contaminants are collected in an oily condensate, which is disposed. 

Thermal desorption is a physical separation process. Waste is heated to volatilize water and organic contaminants. A carrier gas or vacuum system transports volatilized water and organics to the gas treatment system. The bed temperatures and residence times designed for these systems will volatilize selected contaminants without oxidation. Three types of thermal desorption are available ... 

Treatment Method. Oils and Fats Recovery. The oils and fats recovery system adopted obviously depends on the local circumstances. Typically, the first stage of pretreatment is the use of a physical process to recover the free oils and fats. The most commonly used physical separation process for the removal of free oils and fats are fat traps, tilted-plate separators, and dissolved air flotation units. In addition, centrifuge and electroflotation systems are occasionally used (73). 

Deodorizer distillates obtained during the deodorization of chemical refined soybean oil usually have a significantly higher added value as a result of the high concentration of valuable minor components such as tocopherols and sterols (Table 11). A complex downstream processing of these deodorizer distillates, consisting of a combination of chemical and physical separation processes, finally results in the production of purified tocopherols and sterols. 

Inhibitors are classified into reversible and irreversible types. Reversible inhibition implies that the activity of the enzyme is fiiUy restored when the inhibitor is removed from the system in which the enzyme acts by some physical separative process, such as dialysis, gel filtration, or chromatography. An irreversible inhibitor combines covalently with the enzyme so that physical methods are ineffective in separating the two. For example, organophosphorus compounds are... 

The chemistry side of the chemical process industries is concerned with the change of raw materials into products by means of chemical conversions. A single-reacted starting material rarely gives only pure product, so it is usually necessary to use physical separations such as crystallization, filtration, distillation, or phase separation to recover the product(s) from the unreacted starting materials and by-products. By-products are materials other than the product, which are obtained from reacted starting materials. These physical separation processes are often called unit operations to distinguish this from the chemical conversion. Similar features of unit operations may be compared from process to process, unlike chemical conversions [17]. The combination of the chemical conversion step, with all of the unit operations (physical separations) that are required to recover the product of the chemical conversion, is collectively referred to as a unit process. 

Irreversible inhibition occurs when the inhibitor reacts at or near the active site of the enzyme with covalent modification of the active site or when the inhibitor binds so tightly that, for practical purposes, there is no dissociation of enzyme and inhibitor. The latter situation occurs in the case of proteinase inhibitors (see below). Thus, physical separative processes are ineffective in removing the irreversible inhibitor from the enzyme. Irreversible inhibitor reaction is written... 

Another physical separation process makes use of the magnetic properties of certain minerals. Ferromagnetic metals are strongly attracted to magnets. The mineral... 

Table 10.1 lists the principal types of pyrometallurgical processes on which experimental work has been conducted. These have been grouped into physical separation processes, in which no chemical reactions take place, and chemical separation processes. 

In general, material enrichment is conducted by physical separation processes such as density separation, magnetic separation, electrical assortment, and flotation. With complex or poorer ores where physical separation is neither possible nor economically feasible, a wet-chemical process such as leaching, precipitation or extraction is often used. Generally, the wet-chemical processing of the ore preparation is also a substantial part of the hydrometallurgical... 

Since crystallization is a purely physical separation process, neither solid waste nor wastewater is produced. The comparatively small flows of waste air due only to changes in the contents of the plant are taken to the central waste-gas combustion plant. All impurities in the crude naphthalene are concentrated in the residual oil from stage 1 and can be economically utilized in further processing stages. This naphthalene purification process was installed in the Castrop-Rauxel works of VFT, and has produced > 60 000 t/a of pure naphthalene consistently and without problems for more than 10 years. 

MBRs are combined processes in which a biochemical conversion (by the action of a catalyst of biological origin, i.e., an enzyme) and a physical separation process are simultaneously carried out. 

Hollow fibre membranes can be considered as practical and cheaper alternatives than conventional chemical and physical separation processes. They ofifer high packing densities and they can withstand relatively high pressure owing to their structural integrity. In this contest, they allow flexibility in system design and... 

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